Unsaturated fatty acids, in particular, higher unsaturated fatty acids, contained in fats and oils of food may cause peroxidation, and hence deterioration of food mainly results therefrom. These higher unsaturated fatty acids tend to undergo oxidation, and peroxides or free radicals thus produced not only bring about loss of flavor but also are considered to be the cause of aging (or senility) or carcinogenesis in living bodies.
In order to inhibit the oxidation of unsaturated fatty acids, it has been attempted to add a synthetic antioxidant such as 2,6-di-tert-butyl-4-methylphenol (BHT) or butyl hydroxyanisole (BHA), and a naturally occurring antioxidant such as tocopherol, a flavonoid derivative or a lignan analogue.
The synthetic antioxidant such as BHT or BHA has a problem in its safety, and hence the amount of use thereof or the objects to which it is used are restricted. Accordingly, it has been sought to provide an antioxidant which is safer and is free from any restriction of use. The BHT and BHA not only have the problem in safety but also have a question about efficacy in respect of eicosapentaenoic acid (EPA), docosahexaenoic acid (DHA) or fish oil. Thus, it has been sought to develop a remarkably effective antioxidant for these higher unsaturated fatty acids.
On the other hand, although the natural antioxidant is considered safer than the synthetic one, it is chiefly tocopherol that is actually used, and other natural antioxidants have been little used alone.
However, the tocopherol can not have a sufficient antioxidant effect against fats and oils that contain a higher unsaturated fatty acid like fish oil and tend to undergo oxidation.
As a natural antioxidant material, a soybean protein containing a flavonoid derivative, a sesami protein containing a lignan analogue, and a wheat protein as well are known to have an antioxidant effect. There, however, is no report on an antioxidant obtainable from other protein materials.